The modulation schemes used in the optical transmission field is making transition from the dual polarization quadrature phase shift keying (DP-QPSK) scheme to multi-level quadrature amplitude modulation (QAM) scheme. In a PSK scheme, symbol information represented as signals of “0” or “1” is transmitted. In contrast, in a 16-QAM scheme, symbol information represented as I and Q signals each having four values is transmitted. Accordingly, an optical transmission apparatus employing the 16-QAM scheme requires low distortion characteristics in a signal processing unit such as a digital-to-analog converter (DAC), an amplifier and an electrical-to-optical (E/O) converter. Therefore, in the optical transmission field, there is a need for a circuit that can compensate for the distortions of an output waveform in a signal processing unit at the electrical domain.
In the present state of the art of optical transmission area, there have been known a frequency correction in which an equalizer (EQ) compensates for the falling of frequency in an output waveform by applying reverse frequency characteristics, and a non-linear correction in which a linearizer compensates for the saturation characteristics of a DAC or an amplifier by applying reverse saturation characteristics. In addition, by using an EQ or a linearizer, it is possible to correct a distortion in an output waveform caused by frequency characteristics or saturation characteristics of a signal processing unit such as an amplifier and a DAC.
In a distortion compensation circuit used in radio frequency transmission field, while the transmission signal is in the frequency range of 20 MHz, the operation frequency is approximately 200 MHz, which is ten times the frequency range. Nonetheless, it is relatively easy to prepare a circuit having the operation frequency of such level. Accordingly, since a sufficient operation frequency can be obtained in the distortion compensation circuit used in the radio frequency transmission field, the frequency compensation can be implemented simply by extracting a non-linear distortion in a high power amplitude (HPA) to compensate for the non-linear distortion.
Related technologies are disclosed in, for example, Japanese National Publication of International Patent Application No. 2010-518660, and Japanese Laid-Open Patent Publication No. 11-345054.